A satellite based positioning system is used to determine a position of a receiver and typically includes satellite control facilities, a plurality of satellites, the receiver, and one or more local or regional ground stations. Each of the satellites transmits a signal that contains a code and certain prescribed information useful to the receiver in determining its position. The receiver synchronizes itself to the codes of at least four satellites and uses the information in the signals from these satellites in order to perform a triangulation like procedure so as to determine its coordinates and time offset with respect to a reference, such as the center of the Earth and the GPS standard time.
The receiver is not constrained to a specific location and, therefore, represents a variable position. Indeed, the purpose of the satellite based positioning system is to make it possible for the receiver to determine its position regardless of the location of the receiver. On the other hand, the local or regional ground station is in a fixed location and is used to monitor the signals transmitted by the satellites. The signals transmitted by the satellites can be adversely affected, for example, by atmospheric conditions which can lead to improper position determinations by the receiver. The ground station, therefore, notifies the receiver of any necessary signal corrections to allow the receiver to make more accurate position calculations. This arrangement is referred to as differential positioning.
The ground station of the present invention also monitors the signals transmitted by the satellites in order to detect faults within the satellites. For GPS, these faults are specified by the FAA who imposes stringent requirements to protect users against positioning system signal faults. A set of test waveforms has been chosen by the FAA to represent at least some of the more egregious faults. These waveforms are used for certification testing of the ground station equipment.
The prior art determines faults by comparing conventional code tracking discriminators at different correlator spacings. As shown in FIG. 1, a correlation curve is established by correlating the code received from a satellite with a suite of code references which are time shifted replicas of the code transmitted by that satellite. For example, seven correlation measurements may be calculated as shown in FIG. 1. The in-phase measurement IP represents the amount of correlation between the received code and a reference code that has a zero time shift with respect to the received code (this measurement is referred to as punctual). The in-phase measurement IE1 represents the amount of correlation between the received code and a reference code that has a first predetermined time shift so that it is early with respect to the received code. The in-phase measurement IL1 represents the amount of correlation between the received code and a reference code that has a second predetermined time shift so that it is late with respect to the received code. Similarly, the in-phase measurement IE2 is derived using a third predetermined time shift, the in-phase measurement IL2 is derived using a fourth predetermined time shift, the in-phase measurement IE3 is derived using a fifth predetermined time shift, and the in-phase measurement IL3 is derived using a sixth predetermined time shift. The magnitude of the first predetermined time shift may be equal to the magnitude of the second predetermined time shift, the magnitude of the third predetermined time shift may be equal to the magnitude of the fourth predetermined time shift, and the magnitude of the fifth predetermined time shift may be equal to the magnitude of the sixth predetermined time shift. It is assumed that all measurements are normalized such that the measured correlation is a function of the time shifts only and not the absolute power of the received satellite signal.
First, second, and third discriminators are then formed according to the following equations:d1=(IL1−IE1)IP d2=(IL2−IE2)IP d3=(IL3−IE3)IP These discriminators are thereafter compared to each other through the formation of quantities d1,2, d1,3, and d2,3 according to the following equations:d1,2=|d1−d2|d1,3=|d1−d3|d2,3=|d2−d3|The quantities d1,2, d1,3, and d2,3 are compared to corresponding thresholds D1,2, D1,3, and D2,3 such that, if the first discriminator d1,2 exceeds D1,2, if the second discriminator d1,3 exceeds D1,3, or if the third discriminator d2,3 exceeds D2,3, a fault is assumed to exist. During normal operation of the global positioning system, this test is performed on the signals received from each of the satellites. During certification, a test is to be performed using each of the test waveforms chosen by the FAA in order to prove that fault detection occurs.
At least one of the problems with this method is that it is requires six correlators in order to determine the three quantities d1,2, d1,3, and d2,3 which is too much hardware for the amount of useful data being provided.
It is also known for ground stations to determine faults by scanning the whole correlation peak (i.e., the portion of the correlation curve around the punctual in-phase measurement IP) in order to determine whether the peak varies from some prescribed norm by a predetermined amount. However, this fault detection arrangement requires a substantial amount of computing power and it lacks accuracy.
A third method in the prior art uses the following ratios between the measurements IE3, IE2, IE1, IL1, IL2, and IL3:             r              E3        ,        E2              =          IE3      IE2                  r              E3        ,        E1              =          IE3      IE1                  r              E3        ,        L1              =          IE3      IL1      Each of these ratios is compared to a corresponding predetermined value.
The present invention is directed to an arrangement which overcomes one or more problems of the prior art.